1. Field of the Invention The field of the invention is Fourier transform infrared spectroscopy and in particular accessories for making specular reflection analysis with such spectrometers.
2. Background Art
Fourier transform infrared spectroscopy (or FTIR) is a technique for studying the composition of matter by measuring the characteristic absorption of specific wavelengths of infrared radiation by the matter. The absorption may be measured with either transmitted or reflected radiation.
In transmission spectroscopy, a beam of infrared radiation of known and time-variant spectral composition is passed through an at least partially transmissive sample. The resulting transmission absorption spectrum is then compared to standard transmission absorption spectra to identify the spectral adsorption characteristics of the sample to permit identification of the sample's constituents.
With opaque samples, the technique of specular reflection spectroscopy may be used. In reflection spectroscopy, a beam of infrared radiation of known spectral and time variant composition is directed against the surface of a planar sample at a predetermined angle of incidence. The spectrum of the energy reflected at an opposing reflection angle, equal in magnitude to the incidence angle, is then measured. As with transmission spectroscopy, the resulting reflection absorption spectrum may be compared to known reflection absorption spectra to reveal information about the composition of the surface or the coating of the surface of the sample.
The spectrum of the reflected beam may also be used to reveal information about the physical structure of the sample, for example, the thickness of a thin film applied to an opaque or reflective substrate. Multiple reflections of the incident radiation in the thin film can cause optical interference effects which sinusoidally modulate the intensity of the measured reflection absorption spectrum as a function of frequency. The "frequency" of this modulation is dependent on the thickness and the refractive index of the film and the angle of incidence of the impinging radiation.
If the angle of incidence of the impinging radiation and the refractive index of the film are known, the thickness of the film may be deduced from the modulation frequency.
If the thickness of the film is such that the resulting reflection spectrum exhibits less than one cycle of modulation across the spectral range of the measurement, the resulting accuracy of the measurement of the film thickness may be poor. Varying the angle of incidence (and reflection) of the incident beam may be used to increase the modulation of the spectrum and hence improve the accuracy of the measurement. Accordingly, the ability to adjust the angle of incidence of the incident radiation is desirable to allow a wide range of film thickness to be evaluated. With very thin films on metallic substrates, the incident radiation beam also may be polarized and adjusted to strike the sample at a high angle of incidence. This further increases the intensity of the spectral features of the reflected beam. The optimum angle for this measurement has been calculated by R. G. Greenler, J. Chem. Physics 44, 10 (1966) and is approximately 88.degree.. The ability to accurately adjust the angle of incidence of a polarized incident beam to 88.degree. may increase the intensity of the spectral features by up to two orders of magnitude.
Standard spectroscopy instruments are conventionally designed for transmissive rather than reflective spectroscopy. For this reason, the spectroscopic light source, the sample chamber, and the detector are ordinarily arranged along a straight path (the "spectroscopic axis"). In a center focus instrument, the beam from the light source ("source beam") is focussed at a focal point centered within the sample chamber. A diverging beam ("detector beam") exits the sample chamber from the focal point and is collected by the detector for analysis. Reflective spectroscopy may be performed with such standard spectroscopic equipment by inserting a specially designed accessory within the sample chamber that will intercept the source beam, divert it to the sample for reflection, and return it after reflection to the detector.
In one design for a prior art specular reflection accessory, as shown in FIG. 1(a), a first transfer mirror 10 reflects the source beam 16 toward a sample 12 removed from the spectroscopic axis 70, at a given angle of incidence .phi.. A second transfer mirror 10' receives the reflected light from the sample 12 at a reflection angle .phi.', of equal magnitude to the incidence angle .phi., and reflects the received light back to the detector. The drawback to this system is that when the accessory is placed in the spectrometer, an extra path length is introduced causing a subsequent defocussing of the detector beam and the loss of signal. The path length of the beam with the accessory inserted is equal to the distance from the transfer mirror 10 to the sample 12 to the second transfer mirror 10' while the distance with the accessory removed is equal to the distance from the transfer mirror 10 to the transfer mirror 10'. This accessory is thus a simple low performance device which is designed to be used at a fixed angle of incidence to the sample.
A second design for a prior art specular reflection accessory is shown in FIG. 1(b). A first transfer mirror 10 directs the source beam 16 away from the spectroscope axis 70 towards a first ellipsoidal mirror 14 which reflects the beam back toward the spectroscope axis 70 and the sample 12. The ellipsoidal mirror has a first focal point at the first transfer mirror 10 and a second focal point on the surface of the sample 12 and hence with rotation of the first transfer mirror 10, the source beam 16 remains directed to the sample 12 by the ellipsoidal mirror 14 with only the angle of incidence .phi. changed. A corresponding second ellipsoidal mirror 14' and second transfer mirror 10' collect the reflected light and return it to the detector for analysis. The two transfer mirrors 10 and 10' must be realigned to change the angles of incidence and reflection while the sample 12 remains stationary. The drawbacks of this design are that the two mirrors must move in precise unison, and that the sample 12 is fixed inside of the instrument which is both inconvenient and limiting as to the size of the sample.
A third design for a specular reflection accessory, shown in FIG. 1(c), uses a rotatable sample holder 13 joined at a right angle to a sample mirror 15. A first transfer mirror 10 directs the source beam 16 along a deflection path 16' toward the sample 12 which is displaced away from the spectroscope axis 70. The reflected beam 18' from sample 12 is in turn reflected by the sample mirror 15 along a return path 18' toward the spectroscope axis 70. As before, the second transfer mirror 10' directs the reflected beam to the detector for analysis. As a result of the geometry of the sample holder 13 and sample mirror 15, the reflected beam from 18' the sample 12 is parallel to the deflection path 16'. This parallelism of the return path 18' with the deflection path 16' is independent of the angle of the sample 12 with respect to the deflection path 16'. Hence the angle of incidence .phi. and reflection .phi.' may be changed by rotating the sample holder 13 without adjusting the transfer mirrors 10 and 10'. The drawbacks to this design are the limitation of the size of sample 12 imposed by sample holder 13, and the need to clamp the sample 12 against the sample holder 13 to hold it during rotation.